Method for reducing nitrogen oxide on the primary side in a two-stage combustion process

ABSTRACT

Method of reducing the nitrogen oxide formation (NO x ) on the primary side and of at the same time avoiding the formation of nitrous oxide (N 2 O) and ammonia slip (NH 3 ) in the exhaust gas of a two-stage combustion process and of improving the slag balance, comprising a fixed-bed burn-out zone, through which an oxygenous primary gas flows, above a fuel bed and a downstream exhaust-gas burn-out zone into which oxygenous secondary gas is additionally introduced. The object is to propose a simple and reliably controllable method for reducing nitrogen oxide formation on the primary side in combustion plants, for example grate combustion plants, with considerably higher efficiency, wherein no additional pollutants are produced or the utilization of the energy of the heat content of the combustion gases is only marginally impaired. The object is achieved in that the calorific value of the exhaust gas between the fuel bed surface and upstream of the exhaust-gas burn-out zone is reduced in such a way that an average calorific value of less than 1 MJ/m 3  occurs, and the temperature of the fuel bed surface is at least 950° C. until the exhaust gas leaves the exhaust-gas burn-out zone, and the gas temperature above the fuel bed in the region of the rear grate half is more than 1000° C.

The invention relates to a method for reducing the formation of nitrogenoxide (NO_(x)) on the primary side while concurrently avoiding nitrousoxide (N₂O) and ammonia slip (NH₃) in the offgas of a two-stagecombustion process, comprising a fixed-bed burn-out zone through which aprimary gas containing oxygen flows and a downstream offgas burn-outzone into which a secondary gas containing oxygen is additionallyintroduced, according to the first patent claim. The invention alsoserves to improve the quality of the slag by reducing the chlorideconcentration in the grate ash.

In combustion processes, especially in grate-fired furnaces, the thermalformation of nitrogen oxide (NO_(x) formation) from nitrogen in the airis negligibly small due to the relatively low temperature level. Whenfuels containing nitrogen are burned in these furnaces, nitrogen oxidesare formed largely from the nitrogen bonded in the fuel.

The burn-out of solid fuels such as waste, biomass or coal on combustiongrates can be divided in an idealized manner into the consecutivepartial processes of drying, de-gassing and burn-out of the solidcarbon. In industrial grate-fired furnaces, these partial processesoverlap. During the de-gassing phase, not only the hydrocarbons but alsothe nitrogen compounds—especially NH₃ (ammonia) and HCN (hydrogencyanide)—that are formed primarily from the fuel nitrogen are releasedinto the offgas. The concentration of hydrocarbons in the offgasdirectly above the grate, particularly in the area of the maincombustion zone of the incineration system, is so high that the amountof oxygen fed locally there via the primary air is not sufficient tobring about a complete burn-out of the offgas. The offgas exiting fromthe combustion bed in this zone has high offgas temperatures and ispractically oxygen-free. Under these conditions, carbon monoxide (CO)and hydrogen (H₂) are formed via gasification reactions. Consequently,it is also in this area that the highest concentrations are found ofhigh-heating-value offgas components such as hydrocarbons, carbonmonoxide or hydrogen, along with the nitrogen species that are formedprimarily from the fuel nitrogen, mainly ammonia (NH₃) and hydrogencyanide (HCN) as well as, in much smaller quantities, organic compoundscontaining nitrogen such as, for instance, pyridine and aniline.

Normally, in the case of the above-mentioned incomplete combustion dueto a lack of oxygen, after-burning is initiated by adding secondary airto the still high-heating-value offgas. This gives rise to very hightemperature peaks locally, whereby NO or N₂ are ultimately formed fromthe above-mentioned NH₃ and HCN compounds under the oxidizing conditionsvia complex reactions during the offgas burn-out. The objective is tomodify the control of this process in such a way that the primarynitrogen species NH₃ and HCN are completely degraded and that N₂ ispreferably formed as the final product, at the expense of the formationof nitrogen oxide while, at the same time, avoiding the formation ofN₂O.

[1] discloses the dependence of the burn-out rate on the primary airvolume during the burn-out of solids. Depending on the properties of thefuel, particularly on the heating value, the burn-out rate displays amaximum at a certain primary air volume. A further increase in theprimary air volume beyond this maximum, in contrast, causes thecombustion bed to cool down. The reduced or delayed release of volatilefractions from the fuel that is associated with the cooling as well asthe dilution of the combustion gases with the fed-in primary air cause alocally reduced release and thus a diminished concentration ofhydrocarbons, CO and H₂.

[2] and [3] disclose this measure in conjunction with the additionalinformation that a high feed of primary air with a concurrent low feedof secondary air (constant sum of primary and secondary air)fundamentally lead to low NO values in the combustion offgases.

The injection of water for purposes of reducing the formation of PCDD/Fin waste incineration plants is proposed in [4]. An advantage ispostulated to be the reduction in NO_(x) formation due to thetemperature drop caused by the injection of water. All of the offgastemperatures cited in [4] refer to the area before the gas enters theoffgas burn-out zone and they are between 800° C. and 950° C. or 970° C.[1472° F. and 1742° F. or 1778° F.]. Unfortunately, however, detailedNO_(x) values and NO_(x) reduction rates are not given. Moreover, thereis no information about other pollutants containing nitrogen, especiallyN₂O and NH₃.

Lowering the offgas temperatures to below 950° C. [1742° F.] after ithas left the offgas burn-out zone, however, leads to incompletedegradation of the primarily formed NH₃ (ammonia) and also to theformation of N₂O (laughing gas), both of which escape into theatmosphere as strong greenhouse gases if they are not treated within thescope of additional process steps, for example, with catalysts.

The temperature reduction to 800° C. to 950° C. [1472° F. to 1742° F.]mentioned in [4] resulting from the addition of water, however, causes adrop in performance, even if the heat is utilized downstream, forinstance, in order to heat up a boiler.

The same effect is achieved by moistening the fuel, which reduces theheating value of the fuel. The maximum of the burn-out rate is alreadyexceeded at small primary air volumes. The burn-out of the solidsextends over a long grate area, whereby the heating values of the gas aswell as the offgas temperatures become established at a low level beforethe gas enters the offgas burn-out zone. The above-mentioned effectsoccur here as well.

Before this backdrop, it is the objective of the invention to putforward a simply and reliably controllable method that can be used moreefficiently to reduce pollutants containing nitrogen, especially theformation of nitrogen oxide, on the primary side in industrial furnaces,for example, grate-fired furnaces. In this context, it is particularlyimportant that the method does not cause the formation of otherpollutants such as, for instance, N₂O, that NH₃ slip is avoided and/orthat the energetic utilization of the heat content of the combustionoffgases and the quality of the slag are not significantly diminished.

This objective is achieved by means of a method having the features ofthe first patent claim. Advantageous embodiments of the method can befound in the subordinate claims.

For purposes of achieving the objective, a method is proposed forreducing the formation of nitrogen oxide on the primary side in atwo-stage combustion process, that is to say, a process comprising afixed-bed burn-out zone and an offgas burn-out zone located downstreamin the offgas exhaust system. Here, the actual combustion of the solidfuel takes place in the fixed-bed burn-out zone, while after-burning ofthe incompletely burned offgas components takes place in the offgasburn-out zone. In such combustion processes, a primary gas containingoxygen is introduced into the fixed-bed burn-out zone and a secondarygas that likewise contains oxygen is introduced into the offgas burn-outzone for purposes of after-burning.

An essential aspect of the invention is a systematic reduction in theheating value of the combustion gases before they enter the offgasburn-out zone, namely, in such a way that, as a result, a significantreduction in the formation of nitrogen oxide is achieved but, on theother hand, the offgas temperature is not concurrently lowered locallyto such an extent as to cause the formation of pollutants such as, forinstance, N₂O, or an incomplete degradation of NH₃. Towards this end, itis absolutely necessary to precisely maintain a certain state of theoffgas. On the one hand, in order to prevent the formation of nitrogenoxide, the offgas or even only parts of it must not exceed a certainlimit heating value, preferably 1.5 MJ/m³, more preferred 1.0 MJ/m³ and,on the other hand, the temperature of the offgas after it has left theoffgas burn-out zone must not fall below 1000° C. [1832° F.], preferablybelow 980° C. [1796° F.], more preferred 950° C. [1742° F.], forpurposes of limiting pollutants containing nitrogen, especially N₂O andNH₃, not only integrally but also in certain areas. Consequently, it isnot only of crucial importance to systematically regulate or set theheating value and the temperature of the offgas by means of suitablemeasures, but also to attain a systematic homogenization by means ofthese measures.

One possibility is to inject a water-gas mixture upstream from theoffgas burn-out zone. This translates into a systematic reduction in theheating value of the offgas immediately after the fixed-bed combustion(on the combustion bed) with a concurrent homogenization of the gasesbetween the combustion bed and the offgas burn-out zone, in other words,not of the heating value of the solid fuel itself. The injection ispreferably carried out in the form of a free jet characterized by alower volume flow on the one hand and by a high velocity on the otherhand.

The injection of a water-gas mixture advantageously directly affects theheating value of the offgas and not the solid fuel, namely, not only interms of a reduction of the heating value but also in terms of ahomogenization of the heating value.

After all, a high heating value of the offgas in an area directly abovethe combustion bed correlates with the magnitude of the formation ofnitrogen oxide. In this context, the above-mentioned high NO formationrate is caused by the maxima of the axial concentration profiles as wellas by a broad distribution of these maxima of the high-heating-valueoffgas components, namely, C_(n)H_(m) (hydrocarbons), CO and H₂, that isto say, a high integral mean value over the grate length. Hence, thepresent invention describes a suitable technical measure to both reduceand homogenize the heating value of the offgas before the burn-out ofthe offgas, thereby drastically reducing the maxima and the breadth ofthe distribution and thus minimizing the formation of NO.

If the combustion process takes place in a grate-fired furnace, the fuelcontinuously passes through the entire fixed-bed burn-out zone on agrate serving as the combustion bed, said fixed-bed burn-out zone beingdivided into individual fixed-bed areas arranged one after the other.The front half of the grate constitutes the fixed-bed areas throughwhich the fuel passes first while, as the combustion proceeds, the fuelis transported into subsequent fixed-bed areas of the rear half of thegrate, and from there to an outlet for the solid combustion residues.Here, the fixed-bed burn-out zone is arranged in a combustion chamber,and each of the combustion bed areas is provided with its own feed ofprimary gas. The secondary gas containing oxygen is fed into the offgasburn-out zone, preferably in a shared offgas exhaust system for all ofthe combustion bed areas. The water-gas mixture is injected in the formof a free jet directly into the combustion gas above the surface of thecombustion bed into the combustion chamber, in other words, upstreamfrom the offgas burn-out zone, whereby the jet axially penetrates all ofthe combustion bed areas and comes into contact with and mixes thecombustion gases immediately after they are formed.

Fundamentally suitable water-gas mixtures are all mixtures consisting ofwater or aqueous solutions with a gas such as, for example, a water-air,water-offgas or water-steam mixture. Within the scope of theseinventions, aqueous solutions can also contain dissolved re-circulatedpollutants stemming from other cleaning measures (e.g. from cleaningscrubbers).

The water-gas mixture is injected either continuously or in pulses at ahigh velocity or pulse strength, so that the jet axially penetrates thegas space above the combustion-bed surface over all of the grate zones.For purposes of generating the jet, it lends itself to use two-fluidnozzles at a jet angle smaller than 15°, preferably between 3° and 10°.

Fundamentally, however, the water and gas fractions can also be injectedseparately by means of their own one-fluid nozzles, whereby, with an eyetowards the above-mentioned homogenization of the combustion gases, theinjection should be configured in such a way as to ensure that the twoone-fluid jets strike each other and mix with each other as well withthe combustion gases in the combustion space.

The gas fraction of the water-gas mixture injected via the jet shouldnot exceed 10% of the total volume of combustion air introduced, whichconsist essentially of the primary air stream and the secondary airstream. A higher fraction causes, for example, a higher release rate ofdust into the offgas. A fundamental limitation also applies to theintroduced mass flow of the aqueous fractions. An increasing mass flowresults in an increasing cooling of the combustion gases and, above agiven level, this can detrimentally affect or even extinguish theburn-out of the offgas. Cooling the offgas by adding water generallyleads to a reduced energetic utilization of the offgas heat in the caseof steam generation and consequently should be kept to a minimumwhenever possible.

The temperatures upstream from the offgas burn-out zone should always beabove 970° C. [1778° F.] and, downstream from the offgas burn-out zone,above 950° C. [1742° F.], so that no undesired pollutants such as thegreenhouse gas N₂O or a slip of primarily formed NH₃ can occur in theburned-out offgas.

Below a temperature of 950° C. [1742° F.] downstream from the offgasburn-out zone, the N₂O concentration rises exponentially as thetemperature drops. N₂O is a strong greenhouse gas and should thereforebe avoided. Above 950° C. [1742° F.], it is also ensured that theprimarily formed NH₃ is practically completely degraded in the offgasburn-out zone.

Preferably, the amount of water to be fed in via the jet is determinedand regulated by the NO concentration permitted (for example, statutorylimit values) in the offgas downstream from the offgas burn-out zone,that is to say, indirectly via the mean temperature of the combustiongas (offgas) downstream from the offgas burn-out zone in the combustionchamber. In this context, a minimum temperature of 950° C. [1742° F.] inthe offgas downstream from the offgas burn-out zone demarcates the upperlimit of the mass flow of water.

Another alternative or additional measure to lower the heating value ofthe offgas while concurrently ensuring one of the above-mentionedminimum offgas temperatures comprises setting the primary gas feed insuch a manner that a combustion stoichiometry between 0.6 and 1.2,preferably less than 1.0, more preferred between 0.7 and 0.9, isestablished in the primary combustion zone. The minimum volume ofcombustion air and the volume of primary air can be approximatelycalculated from the offgas composition (e.g. CO₂, O₂, H₂O) and from theoffgas volume.

As an alternative to this, another measure for the above-mentionedpurpose involves a systematically adjustable and/or regulatabletransport speed in the combustion bed, whereby, in the front half of thegrate, the transport speed is preferably at least 50% higher than in therear half of the grate, whereby the dwell time of the solid (fuel) onthe grate is dimensioned such that more than 99% of the grate ash burnsout. The basic idea behind this measure is to control the offgasemission generated when a material is burned in a fixed bed and todistribute this emission over the combustion bed surface in such a waythat the offgas has a low heating value above each of the combustion bedareas. In this context, this measure distributes the release ofhigh-heating-value gases over a larger grate surface area, thus markedlyreducing the maximum value of the axial heating value profile of theoffgas above the combustion bed. This spatial extension of the releaseof high-heating-value gases improves the gas burn-out already in thecombustion bed with the primary air fed in since more oxygen is locallyavailable for the oxidation (larger grate area−m³ of air/m² of gratesurface area=constant), thus bringing about a reduction in the integralmean value of the axial heating value profile.

In all cases, low heating values in the offgas (relative to the meanvalue and to the maximum value of the offgas heating value profile inthe cross section of the stream) between the combustion-bed surface andbefore the addition of secondary air fundamentally correlate with lowNO_(x) emission values. Therefore, efforts should be aimed at attaininggenerally low gas heating values before the addition of secondary air,whereby the measures proposed above, either on their own or incombination with each other, ensure a low release of dust from thecombustion bed as well as a good burn-out of the slag and offgas. Inparticular, this makes it possible to achieve low NO_(x) emission valueswithout a significant increase in the formation of N₂O, whereby theabsence of a slip of primarily formed NH₃ presupposes sufficiently hightemperatures in the offgas burn-out zone when said secondary air isadded.

The invention and its advantageous embodiments preferably fulfill thefollowing basic conditions:

-   The primary air ratios (stoichiometry) are set below 1.0, preferably    between 0.7 and 0.9, in order to attain a low release of dust.-   The secondary gas is set in such a way that an oxygen excess of at    least 6%, preferably approximately 10%, remains downstream from the    offgas burn-out zone, as a function of the fuel heating value and of    the fuel moisture.-   The total dwell time of the fuel on the grate is dimensioned in such    a way that a good burn-out of the slag is ensured, whereby a higher    transport speed is established in the front half of the grate than    in the rear half of the grate.-   The axial mixing of the combustion gases in the combustion chamber    by means of a small amount of water/air is preferably carried out    with finely dispersed water using a two-fluid nozzle. The free jet    from a liquid/gas mixture penetrates the combustion space at a high    pulse level in the axial direction (that is to say, usually    extending horizontally and over all areas of the combustion bed). As    a result, the offgas is mixed and the heating value is lowered in    the combustion space.-   The water volume of the water-gas jet is regulated as a function of    the ascertained, preferably measured, NO_(x) concentration in the    offgas downstream from the offgas burn-out zone or downstream from    the boiler.-   The maximum water volume is limited by the ascertained, preferably    measured, minimum offgas temperature of 950° C. [1742° F.]    downstream from the offgas burn-out zone. Here, the temperature    upstream from the offgas burn-out zone must not fall below 970° C.    [1778° F.].-   The loss of the heat quantity needed for downstream heat utilization    in a boiler remains moderate with a low addition of water,    preferably below 50 g/Nm³ of offgas, more preferred below 30 g/Nm³    of offgas.

The invention is described in greater detail below on the basis ofembodiments and of the figures cited below. The following is shown:

FIG. 1—a cross section of a conventional grate-fired furnace with fourcombustion bed areas P₁ to P₄;

FIGS. 2 a to 2 f—the measured axial concentration profiles of O₂, CO₂,H₂O, CO, organic carbon compounds (sum S of organic C) and H₂ in theoffgas above the combustion bed of a conventional grate-fired furnace;

FIGS. 3 a and 3 b—measured nitrogen oxide concentrations 10 in thecombustion bed as a function of the offgas heating values 11 and 12 (a)or of the stoichiometry 17 (primary air ratio) and moving grate speed 18(b);

FIG. 4—measured values of the laughing gas concentration (N₂O) 19 and ofthe nitrogen oxide concentration (NO) 10 in the offgas as a function ofthe offgas temperature 20 as it leaves the offgas burn-out zone;

FIG. 5—a cross section of a grate-fired furnace with four combustion-bedzones and a two-fluid nozzle;

FIG. 6—the nitrogen oxide concentrations in the offgas of a grate-firedfurnace according to FIG. 5, ascertained within the scope of a series ofexperiments;

FIGS. 7 a and 7 b—the nitrogen and laughing gas concentrations in theoffgas as a function of the offgas temperature downstream from theoffgas burn-out zone; as well as

FIGS. 8 a to 8 c—the curves of the water concentration, of the formationof nitrogen oxide, of the formation of laughing gas as well as of thetemperature distributions above the combustion bed ascertained as afunction of the time within the scope of experiment example 4.

A conventional grate-fired furnace, as depicted in FIG. 1, consistsessentially of a combustion bed 1 on a firing grate 2 in a combustionchamber 3 having an inlet 4 for fuel, an outlet 5 (see fuel transportdevice 32) for slag or other solid combustion products as well as anoffgas burn-out zone 6 downstream from the combustion chamber in theoffgas exhaust system. The combustion bed 1 consists essentially of asolid fuel. The combustion chamber 3 covers all of the combustion bedareas P₁ to P₄ through which the fuel in the combustion bedconsecutively passes and a primary gas feed 7 containing oxygen for eachindividual combustion bed area flows through the grate in each of thecombustion bed areas P₁ to P₄. In this context, P₁ and P₂ form the fronthalf of the grate, while P₃ and P₄ form the rear half of the grate. Theabove-mentioned injection 9 of secondary gas containing oxygen takesplace in the downstream offgas burn-out zone 6 in the offgas exhaustsystem.

The combustion 8 of the solid fuel (only shown by a symbolic flame inFIG. 1) takes place essentially in the area of the combustion bed P₂and, by nature, different combustion states occur in the combustion bedareas P₁ to P₄, that can be ascribed particularly to the progress of thecombustion and the temperature of the fuel. FIGS. 2 a to 2 f provideexamples of measured concentration profiles of the offgas componentsoxygen O₂ (a), carbon dioxide CO₂ (b), water H₂O (c), carbon monoxide(d), organic hydrocarbons compounds (e) as well as hydrogen H₂ (f) inthe combustion chamber 3 directly above the combustion bed 1, plottedover the combustion areas P₁ to P₄. During the combustion, adegasification of the volatile fuel constituents occurs, especially ofhydrocarbons C_(n)H_(m) (see FIG. 2 e). The hydrocarbon concentration inthe offgas in the area of the main combustion zone (combustion bed areaP₂) is so high here that the locally fed-in oxygen (FIG. 2 a) is notsufficient to bring about a complete offgas burn-out. The oxygenconcentration here can even drop all the way to zero. This is the placewhere the highest concentrations of high-heating-value offgas componentstend to be found, namely C_(n)H_(m), CO and hydrogen (FIGS. 2 d, 2 e and2 f), namely, together with the primary nitrogen species (NH₃, HCN andsmall amounts of hydrocarbons containing nitrogen). Water (FIG. 2 c) isformed due to evaporation or drying, or else due to the partialcombustion of hydrocarbons and, in the area upstream from and extendingto the main combustion zone, tends to exit from the combustion bed, thendropping to a minimum in the combustion bed areas (P₄) that follow.Carbon dioxide (FIG. 2 b) is formed during the combustion in all of thecombustion bed areas approximately proportionally to the intensity ofthe burn-out.

FIGS. 3 a and 3 b each show a characteristic diagram of the nitrogenoxide concentration 10 (in mg/Nm³, standardized to 11% O₂) in the offgasdownstream from the boiler, ascertained in a waste incineration pilotplant (TAMARA) as a function of several influencing factors. The offgastemperatures in FIG. 3 b downstream from the offgas burn-out zone (afterthe addition of secondary air) were set at a constant value ofapproximately 1050° C.±40° C. [1922° F.±72° F.] in both cases.

On the basis of a large number of experiments with different combustionparameters such as the heating value of the solid fuel, the primary airratio and the grate kinematics, FIG. 3 a depicts a characteristicdiagram of the nitrogen oxide concentration 10 (in mg/Nm³, 11% O₂) inthe offgas downstream from the boiler, as a function of the offgasheating values above the combustion bed, namely, averaged over the meanheating value Hu_(mean value) 11 (in MJ/m³), that is to say, over thegrate length (integral mean value) as well as over the maximum heatingvalue Hu_(maximum value) 12 (in MJ/m³). All of the combustion parametersinfluence the axial heating value profiles of the gas above thecombustion bed. The maximum values and the breadth of the heating valueprofiles of the gas correlate with the NO_(x) concentrations. The lowestNO values are observed at low mean heating values and low maximumheating values. Therefore, the objective is to employ suitable measuresto set low gas heating values in the offgas between the combustion bedsurface and before the secondary air injection.

FIG. 3 b depicts the characteristic diagram of the nitrogen oxideconcentration 10 (in mg/Nm³, 11% O₂) in the offgas downstream from theboiler, as a function of the stoichiometry (dimensionless primary airratio 17) as well as of the moving grate speed 18 in cm/min, which isset at the same value in all of the grate zones (household waste Hu 7 to8 MJ/kg). This characteristic diagram also has an unambiguous area withan extremely low nitrogen oxide concentration whereby, in contrast tothe rise in the nitrogen oxide concentration in FIG. 3 a, the rise inthe characteristic diagram shown here does not take place linearly butrather approximately exponentially. As the stoichiometry increases, theformation of nitrogen oxide advantageously drops steadily. However,stoichiometries above 1.0 should be avoided due to the undesiredincreasing release of dust into the offgas that occurs in this area anddue to the associated contamination of the boiler or the increasedaccumulation of fly ash in the dust extractor.

FIG. 4 depicts the laughing gas concentration 19 (N₂O formation inmg/Nm³, 11% O₂) as a function of the offgas temperature 20 in ° C.,downstream from the offgas burn-out zone. Below a limit temperature ofapproximately 950° C. [1742° F.] downstream from the offgas burn-outzone, a significant rise in the concentration of laughing gas can beexpected. Therefore, when the heating value of the offgas is lowered forpurposes of reducing the emission of pollutants containing nitrogen, theoffgas temperature downstream from the offgas burn-out zone should beset higher than the above-mentioned limit temperature so that thereduced emission of nitrogen compounds and thus the higher fraction ofbound nitrogen retained in the offgas or fuel are not shifted towards agreater emission of laughing gas.

The reduction of the gas heating values in the offgas takes place withinthe scope of the invention by appropriately setting the airdistribution/grate kinematics and the axial mixing of the offgas streamsfrom the individual combustion bed areas (grate zones) P₁ to P₄ in thecombustion chamber 3 before the addition of secondary gas 9 with theconcurrent addition of water droplets. Technically, this is done bymeans of a two-fluid nozzle 13 with a jet 14 of a water-gas mixturewithin the scope of the embodiment according to FIG. 5; for the rest,the set-up corresponds to the design according to FIG. 1. The two-fluidnozzle 13 is positioned from the rear of the combustion space 3. The jetangle is preferably small, that is to say, less than 15°, preferably<100. At a high pressure, a small air volume flow is injected (maximum10% of the offgas throughput rate, typically 12 to 15 Nm³/h at an offgasthroughput rate of, for instance, 400 Nm³/h above the combustion bed).The resulting high pulse level (product of the weight and velocity ofthe free jet) causes the free jet 14 to penetrate the combustion chamber3 and leads to an intensive mixing of high-heating-value andlow-heating-value gases from the individual combustion-bed zones orgrate zones inside the combustion chamber in the area upstream from theoffgas burn-out zone 6 in the case of the secondary gas injection 9. Dueto the mixing of the oxygen-free and high-heating-value gases from themain combustion zone with the oxygen-rich and low-heating-value gasesfrom the grate zones upstream and downstream from the main combustionzone, the high-heating-value gas components are already partially burnedout before reaching the offgas burn-out zone. The efficiency depends onthe primary air ratio and on the mixing quality. The burn-out causes thecombustion chamber temperature to rise. A two-fluid nozzle 13 can beused to additionally feed finely atomized water into the area of thecombustion 8. This causes the heating value in the offgas to drop,ideally by the evaporation enthalpy of the water droplets. At the sametime, this lowers the temperature downstream from the offgas burn-outzone.

The process of effective mixing in conjunction with a minimum volumeflow of the introduced water-gas mixture can be individually optimizedto the geometry of the combustion chamber by adjusting the primary gasstoichiometry, the grate kinematics and the positioning of the two-fluidnozzle, as an alternative also by means of the above-mentionedindividual nozzles for the water and gas or by several free jet nozzles.

The invention will explained in greater detail below with reference toexperiment examples:

EXPERIMENT EXAMPLE 1

The experiments of this example serve to ascertain the optimalcombustion parameters.

In these experiments, waste having a lower heating value Hu of about 7to 8 MJ/kg was incinerated in the above-mentioned TAMARA wasteincineration pilot plant. The oxygen content in the offgas downstreamfrom the boiler was constant at approximately 10 vol-% dry, the offgastemperatures downstream from the offgas burn-out zone were likewiseconstant between 1050° C. and 1100° C. [1922° F. and 2012° F.]. Areduction in the primary air was compensated for by an appropriatelyregulated increase in the secondary air feed, whereby the oxygen excessdownstream from the after-burning chamber was kept constant. Theexperiments were carried out with three different grate speeds. Theresults are shown in FIG. 3 b.

As the primary air ratio 17 (stoichiometry) and/or the moving gratespeed 18 increase, the nitrogen oxide concentration 10 in the offgasdrops (see FIG. 3 a), but so does the heating value of the combustionoffgases above the combustion bed. In this context, further increasing,for example, the primary air ratio in that area to above 1.0, especiallyat high moving grate speeds, does not bring about any further reductionin nitrogen oxide formation, but causes an undesired, high release ofdust into the offgas. For this reason, primary air ratios above 1.0should be avoided.

A high moving grate speed in all of the grate zones concurrentlytranslates into a short dwell time of the solid fuel in the combustionbed on the firing grate, as a result of which the slag burn-out qualityalso diminishes (disadvantage).

Further experiments have shown that a preferred low rate of release ofnitrogen oxide especially correlates with an increase in the movinggrate speed in the front half of the grate, where the volatileconstituents and the primary nitrogen compounds NH₃ and HCN arereleased. The moving grate speed at the end of the grate does not haveany influence on the formation of NO_(x). The moving grate speed in therear half of the grate correlates especially with the quality of theslag burn-out. In the rear half of the grate, as the moving grate speeddrops, the fuel has more time for a more complete burn-out and thus foran improving slag burn-out quality.

EXPERIMENT EXAMPLE 2

As was the case in the first experiment example, the incineration wascarried out in the TAMARA waste incineration pilot plant. The fuel feedwas set in such a manner that the combustible fraction of the solid wasconstant, as a result of which a constant oxygen fraction in the offgasbetween 11 vol-% and 11.5 vol-% was established in the offgas after ithad left the offgas burn-out zone.

Within the scope of this experiment example, the heating value of thesolid fuel Hu_(fuel) was reduced from 12 to 6 MJ/kg by moistening thefuel. The constant load of combustible constituents (constant carbonload) was compensated for by an increase in the fuel feed correspondingto the increase in moisture. The primary air ratio was set at anapproximately constant 1.0 and, by the same token, the moving gratespeed was set at 10 cm/min in all of the combustion bed areas.

The fuel burn-out, with moisture increase in the waste, takes place overa large area of the grate and it automatically moves downstream. At thesame time, the combustion chamber temperatures drop as the heating valueof the fuel Hu_(fuel) diminishes. The heating value of the gas Hu_(gas)above the combustion bed sinks in parallel (the integral mean value aswell as the maximum value of the axial profile). The measured laughinggas concentration 19 (N₂O) and the nitrogen oxide concentration 10 inthe offgas after it has left the offgas burn-out zone are depicted inFIG. 4 over the offgas temperature 20 measured at the same place. Thetemperature in the offgas downstream from the offgas burn-out zone dropsas the water feed increases. As expected, this brings about asignificant reduction in NO_(x) formation (nitrogen oxide concentration10). However, below about 950° C. [1742° F.], an undesired, significantrise in the laughing gas concentration 19 occurs. Moreover, moisteningthe fuel fundamentally leads to a prolongation of the burn-out time ofthe fuel on the grate. The quality of the slag also drops due to lowertemperatures in the combustion bed at high levels of fuel moisture. Forthis reason, moistening the fuel is not recommended.

A reduction in the NO_(x) formation caused by a lowering of thetemperature in the offgas to below 950° C. [1742° F.] downstream fromthe offgas burn-out zone comes at the expense of a marked increase inthe formation of N₂O. Therefore, temperatures in the offgas below 950°C. [1742° F.] downstream from the offgas burn-out zone should beavoided.

EXPERIMENT EXAMPLE 3

Household waste is incinerated in the above-mentioned TAMARA pilotplant.

FIG. 6 depicts the nitrogen oxide concentrations 10 (NO) and the oxygenconcentrations 15 (O₂) in the offgas downstream from combustion chamber,ascertained experimentally in this series of experiments. The fuel feedis 200 kg/h (household waste Hu=9 to 10 MJ/kg) and the offgas flow rateis approximately 1000 Nm³/h. The stoichiometry of the primary air isapproximately 0.9 and the grate speed is approximately 10 cm/min. In thebasic state A, the combustion system operates without the injection of awater-gas mixture into the combustion chamber, in other words, in amanner corresponding to an installation according to FIG. 1. States Band C are established if the two-fluid nozzle only injects air, namely,12 Nm³/h at 4 bar (B) and 15 Nm³/h at 5 bar (C), which corresponds toabout 1.5% of the total offgas stream. This measure already results in areduction in nitrogen oxide formation in the order of magnitude ofapproximately 17% (from about 300 to 250 mg/Nm³ of NO), whereby thisvalue is only negligibly affected by the amount of pressure and air(within the above-mentioned parameter range). This fact as well as theconstant oxygen volume throughout the entire series of experiments leadto the conclusion that the reduction in nitrogen oxide formation isprimarily brought about by the axial turbulence and thus by thehomogenization of the combustion gases over the above-mentionedcombustion bed areas.

The considerable reduction in the nitrogen oxide formation rate, namelyby up to 66% (from about 300 to about 100 mg/Nm³ of NO) in comparison tothe initial state A, however, is achieved by additionally injectingwater (see FIG. 5). On the basis of the air injection parametersaccording to state B (12 Nm³/h at 4 bar), a two-fluid nozzle is employedto inject an additional 20 l/h (state D), 30 l/h (state E), 40 l/h(state F) as well as 50 l/h (state G) of water. At the same time, thetemperature of the offgas downstream from the offgas burn-out zone alsodrops from over 1000° C. [1832° F.] to below 900° C. [1652° F.]. In thisprocess, a steadily decreasing rate of nitrogen oxide formation wasestablished with a still-unchanged oxygen concentration and increasingwater flow, whereby the increments of the reduction become continuouslyless with the absolute water volume. This means that only slightincreases in the reduction of NO are attained at the expense ofrelatively high energy losses. It was possible to substantiate thereproducibility of this reduction measure by repeatedly interrupting thefree jet, intermittently establishing state A, (see FIG. 6). Therefore,the further reduction in the rate of nitrogen oxide formation from stateB to state D to G can be ascribed exclusively to the additionalreduction in the heating value of the offgases and to the simultaneousmixing (here: not of the solid fuel) brought about by the water feed.

The water feed is preferably regulated via the nitrogen oxideconcentration. In order to avoid excessive energy losses in the offgas(limitation of a downstream heat utilization), the water feed in theoffgas lies below 50 g/m³, preferably below 30 g/m³.

At a high water feed using a two-fluid nozzle (see the measured valueswith water feed 21), the temperature drops and a drastic reduction inthe NO_(x) formation occurs in comparison to the measured values withoutthe addition of water (reference measured values 22) (see nitrogen oxideconcentrations 10 over the offgas temperature 20 in FIG. 7 a). In thisprocess, the water content in the offgas downstream from the offgasburn-out zone (that is to say, also downstream from the boiler)increases by up to 50 g/Nm³, in contrast to which the temperaturedownstream from the offgas burn-out zone drops. No N₂O formation(laughing gas concentration 19) was observed above 950° C. [1742° F.]downstream from the offgas burn-out zone, as is also depicted in FIG. 7b. The formation of N₂O below 950° C. [1742° F.] is only dependent onthe offgas temperature 20 and not on the water content itself. Thetendency towards nitrogen oxide formation is downwards as thetemperature decreases. In comparison to a reference setting with atwo-fluid nozzle, the mixing and water addition, preferably with atwo-fluid nozzle at the same offgas temperature downstream from theoffgas burn-out zone, fundamentally translate into a lower nitrogenoxide concentration (see FIG. 7 a). The cause of this is the reductionin the gas heating value above the combustion bed upstream from theoffgas burn-out zone.

According to the 17^(th) BImSchV (German Federal Immission Control Act),an emission limit value of 200 mg/Nm³ (calculated as NO₂ for a referenceoxygen content of 11% O₂) is permissible, and the proposed method canstay well below this limit. No deterioration of the offgas burn-out wasobserved, which was substantiated by means of a CO measurement. Constantvalues within the range of approximately 1 mg/Nm³ were measured at alltimes. No N₂O was detected above an offgas temperature of 970° C. [1778°F.] before the addition of secondary air and of 950° C. [1742° F.] afterthe addition of secondary air.

The described measure should be combined with additional measures toreduce nitrogen oxide, for example, changing the distribution of primaryair or secondary air (see [2] and [3]). It is very advantageous tocombine high volumes of primary air (primary air stoichiometries withinthe range from 0.6 to 1, preferably in the range from 0.7 to 0.9) withhigh transport speeds of the combustion bed (that is to say, higher thanthe above-mentioned 10 cm/min). Only a combination of these twoparameters can achieve a reduction in the NO concentration from about280 to about 150 mg/Nm³ (i.e. an NO reduction of more than 45%) withoutreducing the energetic utilization of the heat content of the offgas(for household waste having a lower heating value Hu=7 to 8 MJ/kg).

Additional experiments have demonstrated that high fixed-bed transportspeeds (that is to say, higher than the above-mentioned 10 cm/min) areonly necessary in the area of the front half of the grate. The rearareas of the grate can be operated correspondingly slower, so thataltogether, sufficient time is available for the burn-out of theresidual carbon in the grate ash.

EXPERIMENT EXAMPLE 4

Within the scope of this experiment example, household waste (Hu=7MJ/kg) is incinerated in the TAMARA pilot plant at a primary air ratioof approximately 0.65 and with an oxygen fraction in the offgas(downstream from the boiler) approximately at a constant 10 vol-% dry.In contrast to the above-mentioned experiment examples, the grate speedin the individual grate areas is not the same here; it is kept at aconstant 22 cm/min in the combustion bed areas P₁ and P₂ (front half ofthe grate) and further reduced in the rear half of the grate for everycombustion bed area (P₃=11 cm/min, P₄=5 cm/min). The distribution of therelative dwell times of the solid fuel in the above-mentioned combustionbed areas P₁ to P₄ was consequently 12%, 12%, 24%, 52%. On the basis ofthe above-mentioned general relationships, these operating parametersalready yielded a low nitrogen oxide formation value of about 150mg/Nm³, measured downstream from the offgas burn-out zone, and also agood burn-out of the slag.

In FIGS. 8 a and 8 b, the measured nitrogen oxide concentrations 10 inmg/Nm³ and the water concentration 25 in g/Nm³ (FIG. 8 a) as well as thelaughing gas concentrations 19 in mg/Nm³ and the temperatures 24 in ° C.(FIG. 8 b) are plotted over the time of day. At about 9:20 a.m., awater/gas jet was introduced by means of the two-fluid nozzle, which canbe seen from the abrupt drop 29 in the curve of the nitrogen oxideconcentration 26 as well as in the temperature curve 27 (after theoffgas has left the burn-out zone) and in the temperature curve 28,upstream from the offgas burn-out zone. The level of the offgas moisturecurve 30, in contrast, only rises slightly due to the small amount ofwater injected. During the regulation step, the temperatures dropupstream from the offgas burn-out zone to values of approximately 1030°C. [1886° F.] and to values of approximately 950° C. [1742° F.]downstream from the offgas burn-out zone.

Moreover, FIG. 8 c shows the axial temperature distributions above thecombustion bed in the individual combustion bed areas P₁ to P₄, plottedover the time of day 23 of the experiment. The isotherms are eachindicated with their temperatures. The temperatures above the rear halfof the grate of the combustion bed rise markedly at the beginning of andduring the injection 33. This effect has positive consequences for thequality of the slag.

The water volume introduced by means of the installed two-fluid nozzleis regulated by measuring the nitrogen oxide in the offgas downstreamfrom the boiler (that is to say, downstream from the offgas burn-outzone). The control loop is programmed in such a way that the maximumwater amount is limited by a minimum temperature of 950° C. [950° F.](see the temperature curve 27 in FIG. 8 b) in the combustion space. Thetarget value of the regulation is set at 40 mg/Nm³ (see the nitrogenoxide concentration curve 16 in FIG. 8 a). Immediately after theregulation system has been put into operation, the nitrogen oxide valuedrops spontaneously (see the graduated decreases 29). This state ismaintained over a period of time of more than 4 hours. The mean increasein the offgas moisture (curve 30) is very low, averaging about 25 g/Nm³.The fluctuations of the offgas moisture during the regulation phase arecaused by the set regulation parameters (PID controller) and by thebrief fluctuation of the heating value of the fuel, and are notsignificant for the efficiency of the energetic utilization and for theefficiency in reducing the NO_(x). The extremely low nitrogen oxidevalues attained are comparable to those of expensive SCR methods and arefar below the statutory limit values.

When the temperature curve 27 approaches the 950° C. [1742° F.] limit,trace concentrations of up to 2 to 3 mg/Nm³ occur (see FIG. 8 b:laughing gas concentration curve 31). However, the maximum laughing gasconcentrations measured in the described experiment are close to thedetection limit and are negligible.

The mixing of the combustion gases in the combustion space before theaddition of secondary air markedly raises the gas temperature, despitethe addition of water, above the combustion bed in the area of the rearhalf of the grate and, due to the resulting higher gas injection, alsoin the combustion bed in the rear area of the combustion bed. As aresult, the slag is thoroughly sintered there and thus rendered inert,so that this favors the use of these residual materials as aggregates,without the need for any complex and thus expensive after-treatment.This is why this positive side effect of waste incineration isadvantageous.

The concentrations, especially of carbon (TOC), chloride and evensulfate in the slag are markedly reduced due to the temperature increasein the combustion chamber and in the slag bed of the rear combustion bedzones, which also translates into an advantageous reduction in the ratioof PCDD/F formation in the slag.

BIBLIOGRAPHY

-   M. V. A. Horttanainen, J. J. Saastamoinen, P. J. Sarkomaa: Ignition    Front Propagation in Packed Bed of Wood Particles; IFRF Combustion    Journal, Article No. 200003, May 2000, ISSN 1562-479X-   H. Hunsinger, K. Jay, J. Vehlow: Formation of Pollutants during    Municipal Solid Waste Incineration in a Grate Furnace under    Different Air/Fuel Ratios; Proc. IT3 '02 Conference, May 13-17,    2002, New Orleans, La.-   H. Hunsinger, J. Vehlow, B. Peters, H. H. Frey: Performance of a    Pilot Waste Incinerator under Different Air/Fuel Ratios; IT3 '00    Conference, May 8-12, 2000, Portland, Oreg.-   U.S. Pat. No. 5,313,895

LIST OF REFERENCE NUMERALS

-   1 combustion bed-   2 firing grate-   3 combustion chamber-   4 inlet-   5 outlet-   6 offgas burn-out zone-   7 primary gas feed-   8 combustion, flame-   9 secondary gas injection-   10 nitrogen oxide concentration-   11 mean heating value-   12 maximum heating value-   13 two-fluid nozzle-   14 jet, free jet-   15 oxygen concentration-   16 experiment time-   17 primary air ratio-   18 moving grate speed-   19 laughing gas concentration-   20 offgas temperature downstream from the offgas burn-out zone-   21 measured values with the addition of water-   22 reference measured values-   23 time of day-   24 temperature-   25 water concentration, dry-   26 nitrogen oxide concentration curve-   27 temperature curve after the offgas has left the burn-out zone-   28 temperature curve in the offgas burn-out zone-   29 graduated decrease-   30 offgas moisture curve-   31 laughing gas concentration curve-   32 fuel transport direction-   33 injection

1-10. (canceled)
 11. A method for reducing a formation of nitrogen oxideon a primary side of a furnace and reducing or avoiding nitrous oxideand ammonia slip in an offgas of the furnace in which a fuel is burnedin a combustion process having at least two stages, the methodcomprising: passing a fuel consecutively through each of a plurality ofbed areas of a combustion bed of the furnace; feeding, individually toeach of the bed areas, a primary gas including oxygen so as to burn thefuel in the combustion chamber of the furnace; introducing, into adownstream offgas burn-out zone, a secondary gas including oxygen so asto after-burn incompletely burned offgas components formed during theburning of the fuel; and axially mixing partial offgas streams from thecombustion bed areas by injecting a water-gas mixture as at least onefree jet above a surface of the combustion bed upstream from thedownstream offgas burn-out zone so that the at least one free jetpenetrates the partial offgas streams of the combustion bed areas so asto reduce a heating value of the offgas between the surface of thecombustion bed and the downstream offgas burn-out zone.
 12. The methodas recited in claim 11 wherein the furnace includes a grate-firedfurnace, and wherein the bed areas are disposed one after the other andeach have a respective own primary gas feed.
 13. The method as recitedin claim 11, wherein the water-gas mixture includes at least one ofwater-air, water-offgas and water-steam.
 14. The method as recited inclaim 11, further comprising generating the at least one free jet usingat least one two-fluid nozzle at a jet angle equal to or less than 15°.15. The method as recited in claim 11, wherein a volume of the water-gasmixture injected is equal to or lower than 10% of a total volume ofprimary gas and secondary gas.
 16. The method as recited in claim 11,further comprising determining a volume of water in the water-gasmixture injected using a target nitrogen oxide concentration in theoffgas downstream from the offgas burn-out zone.
 17. The method asrecited in claim 11, wherein a volume of water in the water-gas mixtureinjected is less than or equal to 5 mg/Nm³.
 18. The method as recited inclaim 11, wherein the water-gas mixture is injected so as to establish aminimum temperature for the offgas downstream from the offgas burn-outzone of 950° C.
 19. The method as recited in claim 11, wherein a volumeof the water-gas mixture injected is established using a maximum nitrousoxide concentration in the offgas downstream from the offgas burn-outzone.
 20. The method as recited in claim 11, further comprising settingthe feeding of the primary gas so as to establish a stoichiometry ofless than 1 in the combustion chamber so as to reduce the heating valueof the offgas downstream from the offgas burn-out zone.
 21. The methodas recited in claim 20, wherein the setting is performed so as toestablish a stoichiometry between 0.7 and 0.9.
 22. The method as recitedin claim 11, wherein the combustion chamber includes an inlet and anoutlet, and further comprising setting a transport speed in thecombustion bed so that a transport speed of a front half of thecombustion bed disposed adjacent to the inlet is at least 30% fasterthan a transport speed of a back half of the combustion bed disposedadjacent to the outlet, so as to reduce the heating value of the offgasupstream from the offgas burn-out zone to attain a burn-out of the fuelto a residual carbon concentration of less than 1%.
 23. The method asrecited in claim 22, wherein the transport speed of the front half ofthe combustion bed is at least 50% faster than the transport speed ofthe back half of the combustion bed.
 24. An apparatus for reducing theformation of nitrogen oxide on a primary side of a furnace and reduce oravoid nitrous oxide and ammonia slip in an offgas of the furnace, thefurnace being configured to burn a fuel in a combustion process havingat least two stages, the apparatus comprising: a combustion chamberincluding a grate disposed therein; a combustion bed configured toreceive a primary gas including oxygen so as to burn the fuel on thegrate, the combustion bed including a plurality of combustion bed areas,each of the combustion bed areas having a respective feed of the primarygas and a partial offgas stream; an offgas burn-out zone downstream fromthe combustion chamber configured for after-burning of incompletelyburned offgas components; a secondary gas introduction device configuredto introduce a secondary gas including oxygen into the offgas burn-outzone; and an injection device configured to inject a water-gas mixtureas at least one free jet above a surface of the combustion bed upstreamfrom the offgas burn-out zone so that the at least one free jet axiallypenetrates the partial offgas streams of the combustion bed areas. 25.The apparatus as recited in claim 24, wherein the furnace includes agrate-fired furnace, and wherein the bed areas are disposed one afterthe other and each have a respective own primary gas feed.
 26. Theapparatus as recited in claim 24, wherein the water-gas mixture includesat least one of water-air, water-offgas and water-steam.
 27. Theapparatus as recited in claim 24, wherein the at least one free jet isat least one two-fluid nozzle configured at a jet angle equal to or lessthan 15°.
 28. The apparatus as recited in claim 24, wherein theinjection device is configured to inject the water-gas mixture toestablish a minimum temperature for the offgas downstream from theoffgas burn-out zone of 950° C.
 29. The apparatus as recited in claim24, wherein the combustion chamber includes an inlet and an outlet wherea transport speed is set so that a transport speed of a front half ofthe combustion bed disposed adjacent to the inlet is at least 30% fasterthan a transport speed of a back half of the combustion bed disposedadjacent to the outlet, so as to reduce the heating value of the offgasdisposed upstream from the offgas burn-out zone to attain a burn-out ofthe fuel to a residual carbon concentration of less than 1%.
 30. Theapparatus as recited in claim 24 wherein the injection device isconfigured to inject the water-gas mixture so as to reduce a heatingvalue of the offgas between the surface of the combustion bed and theoffgas burn-out zone.